bug#49063: GNOME Music fails to start

[Luis Felipe]

Hi, this happens on Guix System, using Guix 154a4e0 and GNOME Music 3.34.5.

STEPS TO REPRODUCE

1. Click on Activities
2. Click on Show Apps button (nine dots button)
3. Click on the Music app

EXPECTED RESULT

The application starts.

UNEXPECTED RESULT

You can see the application icon displayed next to the GNOME Activities button 
with a loading indicator. But, after some seconds, it disappears and the 
application never starts.

ADDITIONAL INFO

Running the application from a terminal results in the following error:

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$ gnome-music
removed, grl-bookmarks
removed, grl-metadata-store
removed, grl-filesystem
Traceback (most recent call last):
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/bin/..gnome-music-real-real",
 line 153, in <module>
    sys.exit(main())
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/bin/..gnome-music-real-real",
 line 148, in main
    return run_application()
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/bin/..gnome-music-real-real",
 line 135, in run_application
    app = Application('org.gnome.Music')
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/lib/python3.8/site-packages/gnomemusic/application.py",
 line 70, in __init__
    self._player = Player(self)
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/lib/python3.8/site-packages/gnomemusic/player.py",
 line 397, in __init__
    self._gst_player = GstPlayer(application)
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/lib/python3.8/site-packages/gnomemusic/gstplayer.py",
 line 86, in __init__
    self._setup_replaygain()
  File 
"/gnu/store/v9vs6yai5dlyf5avly5ni3xxpk7xhfv0-gnome-music-3.34.5/lib/python3.8/site-packages/gnomemusic/gstplayer.py",
 line 111, in _setup_replaygain
    self._filter_bin.add(self._rg_volume)
TypeError: Argument 1 does not allow None as a value
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Luis Felipe López Acevedo
https://luis-felipe.gitlab.io/